minijail: add seccomp softfail option

Minijail now softfails when seccomp is not implemented
by the OS. However if the USE_seccomp flag is defined,
minijail still hardfails.

BUG=chromium:368071
TEST=Manual: compiled with and without USE flag.
CQ-DEPEND=CL:212960

Change-Id: If2b58ddd5ce51a357bda325d32fe2b3dac6df11f
Reviewed-on: https://chromium-review.googlesource.com/212919
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Tested-by: Utkarsh Sanghi <usanghi@chromium.org>
Commit-Queue: Utkarsh Sanghi <usanghi@chromium.org>
diff --git a/Makefile b/Makefile
index d931df0..40d43bd 100644
--- a/Makefile
+++ b/Makefile
@@ -11,6 +11,9 @@
 ifneq ($(HAVE_SECUREBITS_H),no)
 CFLAGS += -DHAVE_SECUREBITS_H
 endif
+ifneq ($(USE_seccomp),yes)
+CFLAGS += -DUSE_SECCOMP_SOFTFAIL
+endif
 
 all : minijail0 libminijail.so libminijailpreload.so